Abstract

The timely resolution of inflammation prevents continued tissue damage after an initial insult. In the brain the death of activated microglia by apoptosis has been proposed as one mechanism to resolve brain inflammation. How microglia death is regulated after activation is still unclear. We reported that exposure to lipopolysaccharide (LPS) and interleukin-13 (IL-13) together initially activates and then kills rat microglia in culture by a mechanism dependent on cyclooxygenase-2 (COX-2). We show here that activation of the EP2 receptor for prostaglandin E2 (PGE2) mediates microglia death induced by LPS/IL-13, and that EP2 activation by agonist alone kills microglia. Both EP2 antagonists and ROS scavengers block microglial death induced by either LPS/IL-13 or EP2 activation. By contrast the homeostatic induction of heme oxygenase 1 (Hmox1) by LPS/IL-13 or EP2 activation protects microglia. Both the Hmox1 inducer cobalt protoporphyrin (CoPP) and the Hmox1 product carbon monoxide (CO) attenuated microglial death produced by LPS/IL-13. Whereas CO reduced COX-2 protein expression, EP2 activation increased Hmox1 and COX-2 expression at both mRNA and protein level. Interestingly, caspase-1 inhibition prevented microglia death induced either by LPS/IL-13 or low (but not high) concentrations of butaprost, suggestive of a predominantly pyroptotic mode of death. Butaprost also caused the expression of activated caspase-3 in microglia pointing to apoptosis. These results indicate that EP2 activation, which initially promotes microglia activation, later causes delayed death of activated microglia, potentially contributing to the resolution phase of neuroinflammation.